Examples of the electromagnetic wave transmission medium for connecting high frequency devices to each other whose relative position cannot be determined with precision or one or both of which are changed in position include a coaxial line and a flexible waveguide. The coaxial line is frequently used for its excellent flexibility and relatively inexpensive price. However, the diameter of the coaxial line needs to be smaller as the frequency increases, and therefore, problems arise such as an increase in transmission loss, an increase in machining accuracy for maintaining a transmission characteristic, deterioration in durability, and so on. For example, when polytetrafloroethylene (i.e., TEFLON®) is used for an insulator, and a cutoff frequency fc is set to 100 [GHz] in the coaxial line, its inner diameter becomes about 1[mm]. In such a small coaxial line, not only the loss is increased but also a slight mechanical error greatly affects the transmission characteristic.
The flexible waveguide is excellent in terms of the transmission loss prevention. However, because the flexible waveguide has a tube wall part which is required to be formed into a specific shape (for example, a bellows shape, such as in Japanese Utility Model Examined Publication No. Sho 41-018451 and Japanese Utility Model Examined Publication No. Sho 45-018273), the production efficiency is significantly lowered. In addition, for the flexible waveguide to realize a structure in which millimeter waveband exceeding, for example, 30 [GHz] can be used, a complicated and high-level processing technique is required. Also, such a thin flexible waveguide lacks in durability.
In addition to the bellows-shaped metal waveguide, there exists a waveguide having an ellipsoidal cross section, in which thin conductors are tiled on the surface of a dielectric rod (Japanese Patent Application Laid-open No. Hei 08-195605). Such a waveguide is obtained by merely winding a metal tape on the surface of the dielectric rod that has been prepared, or subjecting the dielectric rod to conductive plating. Therefore, there is an advantage in that the waveguide can be manufactured at reduced cost. However, such a waveguide has a large transmission loss and insufficient flexibility. Further, the transmission mode becomes unstable when the waveguide is bent because the cross section is ellipsoidal, resulting in such a problem that the characteristic changes.